This work describes a procedure based on a set of thermally stimulated luminescence measurements coupled to an original theoretical analysis which allows estimating the distribution in energy of carrier-trapped states developing in the bandgap of silica-based optical fiber glasses under ionizing irradiation. This procedure is applied to undoped, aluminum-, phosphorus- and rare-earth-doped silica samples from tailormade optical fiber preforms, after irradiations in two very different conditions. The extracted Densities Of Trapped States (DOTS) always relate to distributions of trapped holes. Within a 1-1.5 eV energy range above the valence band, these DOTS contain the energy levels of well-known intrinsic or dopant-related color centers recognized as major contributors to the radiation-induced attenuation in silica fibers. Long irradiation times strongly impact the DOTS by depleting shallow states and favoring the “condensation” of holes in deep levels. This enhances the density of color centers (deeper than 1 eV) and explains part of the RIA increase with the dose.